According to Classification of International Association for the Study of Pain (IASP), “pain” is defined as “An unpleasant sensory and emotional experience associated with actual or potential tissue damage; or described in terms of such damage” (Non Patent Literature 1).
Pain is generally classified as being acute or chronic. Acute pain is pain that has been present for not more than three months. Acute pain begins suddenly and is sharp in quality in most cases. Acute pain may be caused by many events or circumstances, such as surgery, broken bones, dental treatment, burns or cut, etc. Chronic pain is pain lasting for not less than three months. Common chronic pain includes headache, low back pain, cancer pain, arthritis pain, neuropathic pain, psychogenic pain (pain occurring in the absence of physical cause of pain, such as past disease or injury). Neuropathic pain, which has also been translated into neurogenic pain, is refractory pain resulting from damages or diseases to the peripheral or central somatic sensory nerve system, including diabetic neuropathy, trigeminal neuralgia, and postherpetic neuralgia (Non Patent Literatures 2 and 3).
Pain is a common medical problem, and relief of pain is an important therapeutic goal. Pain is most commonly treated with analgesics. Analgesics are roughly divided into three categories: (1) opioid analgesics; (2) nonopioid analgesics, such as anti-inflammatory steroids, acetaminophen, and dipyrone; and (3) “adjuvant analgesics” (a diverse group of drugs which are known as “drugs that do not have an analgesic action as a primary pharmacological action but may enhance an analgesic effect when used in combination with analgesics and show an analgesic effect in selected circumstances”).
While opioid analgesics provide a strong analgesic effect by acting on the opioid receptor in the central nervous system; their use is limited because of their serious adverse drug reactions and dependency. Although nonopioid analgesics have an analgesic effect, the effect is weak and various adverse drug reactions may be induced. In addition, no therapeutic drug effective for chronic pain such as neuropathic pain associated with diabetic neuropathy, trigeminal neuropathy and herpes zoster has yet been found, and the development of a drug effective for a broad range of pain, including acute pain and chronic pain, has been desired.
Although a variety of analgesics are currently available for the treatment of pain, huge unmet medical needs still exist in pain treatment. Recent reports estimate that an adequate analgesic effect on acute pain is realized only in one of four patient undergoing surgical treatment (Non Patent Literatures 4 and 5). In addition, inadequate treatment of acute pain may lead to a variety of symptoms, including anxiety, depression, insomnia, fatigue, decreased appetite, nausea and vomiting. Further, unrelieved acute pain may progress to chronic pain.
On the other hand, as concerning chronic pain, WHO estimates that 20% of population of the world has some degree of chronic pain. Chronic pain has a significant impact on both direct health-care costs and associated indirect costs (for example, disability payments, lost productivity). Because adequate relief cannot be achieved in approximately 40% of patients with chronic pain, chronic pain is now considered to be a significant public health problem (Non Patent Literature 6).
Effective treatment for acute pain and chronic pain still remains an unmet medical need of many patients. Therefore, it has been strongly desired to develop a therapeutic agent effective for acute pain and chronic pain.
As animal models of acute pain, models for evaluating transient pain, such as tail-flick test, a flinch/jump test, a hot-plate test, a pinch test are known. As a model for acute persistent pain, a formalin test is used. (Non Patent Literature 7)
On the other hand, as models of chronic pain, a rat chronic constriction injury model (CCI model) and the like are known. Classified by the cause of pain, the CCI model is considered to be a disease model corresponding to neuropathic pain. (Non Patent Literature 8)
As sulfamide derivatives having an analgesic effect, low molecular compounds disclosed in Patent Literatures 1 and 2 are known. 1-Indanesulfamide derivatives with an analgesic effect have not been known, however.